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Add a "filter" primop

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Evaluation of a NixOS configuration spends quite a lot of time in the
"filter" function in Nixpkgs.  As implemented in Nixpkgs, this is a
O(n^2) operation, so it's a good candidate for providing a more
efficient (i.e. primop) implementation.  Using it gives a ~10% speed
increase and a significant reduction in the number of evaluations.

Statistics before (on a NixOS system config):

  time elapsed: 1.3258
  size of a value: 24
  environments allocated: 1980939 (50127080 bytes)
  list elements: 14679308 (117434464 bytes)
  list concatenations: 50828
  values allocated: 2098938 (50374512 bytes)
  attribute sets allocated: 392040
  right-biased unions: 186334
  values copied in right-biased unions: 591137
  symbols in symbol table: 18271
  number of thunks: 1645752
  number of thunks avoided: 1921196
  number of attr lookups: 430798
  number of primop calls: 838807
  number of function calls: 1930107

Statistics after:

  time elapsed: 1.17982
  size of a value: 24
  environments allocated: 1543334 (39624560 bytes)
  list elements: 9612638 (76901104 bytes)
  list concatenations: 37434
  values allocated: 1854933 (44518392 bytes)
  attribute sets allocated: 392040
  right-biased unions: 186334
  values copied in right-biased unions: 591137
  symbols in symbol table: 18272
  number of thunks: 1392467
  number of thunks avoided: 1507311
  number of attr lookups: 430801
  number of primop calls: 691600
  number of function calls: 1492502
This commit is contained in:
Eelco Dolstra 2012-08-13 00:28:08 -04:00
parent 62f72eb9e1
commit 4ccd48ce24
2 changed files with 26 additions and 1 deletions
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25
src/libexpr/primops.cc
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@ -906,6 +906,30 @@ static void prim_map(EvalState & state, Value * * args, Value & v)
}
/* Filter a list using a predicate; that is, return a list containing
every element from the list for which the predicate function
returns true. */
static void prim_filter(EvalState & state, Value * * args, Value & v)
{
state.forceFunction(*args[0]);
state.forceList(*args[1]);
// FIXME: putting this on the stack is risky.
Value * vs[args[1]->list.length];
unsigned int k = 0;
for (unsigned int n = 0; n < args[1]->list.length; ++n) {
Value res;
state.callFunction(*args[0], *args[1]->list.elems[n], res);
if (state.forceBool(res))
vs[k++] = args[1]->list.elems[n];
}
state.mkList(v, k);
for (unsigned int n = 0; n < k; ++n) v.list.elems[n] = vs[n];
}
/* Return the length of a list. This is an O(1) time operation. */
static void prim_length(EvalState & state, Value * * args, Value & v)
{
@ -1120,6 +1144,7 @@ void EvalState::createBaseEnv()
addPrimOp("__head", 1, prim_head);
addPrimOp("__tail", 1, prim_tail);
addPrimOp("map", 2, prim_map);
addPrimOp("__filter", 2, prim_filter);
addPrimOp("__length", 1, prim_length);
// Integer arithmetic